Beat egg whites, cream of tartar, and remaining 1/4 teaspoon salt in a large bowl with a mixer at high speed until foamy, about 1 minute. Beat in confectioners' sugar 1 tablespoon at a time; continue beating until soft peaks form. Beat in remaining 1/2 teaspoon vanilla.

Spread meringue over warm pie, sealing to crust's edges.

Bake until lightly browned and set, about 15 minutes. Cool to room temperature on a wire rack before serving.